This invention relates to the field of telephony. More particularly, this invention relates to an adaptive interface for interfacing a telephone instrument, such as a headset, a handset, a modem, a fax modem, a facsimile machine or a teleconferencing device, to telephone set through a four-wire handset port of the telephone set.
A two-wire telephone set includes a base unit connected to a central office of a telephone service provider via a bidirectional two-wire telephone line and also includes a handset connected to the telephone base via a four-wire handset cable. The handset cable has four wires because, for two-way voice communication, the handset includes both a microphone and a speaker, each of which requires a pair of wires. Each pair of wires of the handset cable communicates a uni-directional signal. Typically, the telephone base supplies audio signals to the speaker and a DC biasing voltage to the microphone, while the telephone base receives audio signals from the microphone. A two-wire to four-wire converter included in the telephone set converts the bi-directional central office signals into the four handset signals. In addition, the telephone set includes a ring detector for detecting an AC ring signal provided by the central office and a hook-switch for signalling the central office of the on-hook/off-hook status of the telephone set. The hook-switch controls draw of DC current from the central office by the telephone set, which is sensed by the central office.
Business organizations often utilize a private branch exchange (PBX) for providing telephone service to telephone users within the organization. Telephones located on the desks of the users are each connected to the PBX while the PBX is connected to a telephone service provider via one or more outside lines. The PBX typically includes capability for appropriately connecting incoming calls to the user telephone sets and for connecting outgoing calls from a user telephone set to an outside line. In this way, fewer than one outside line per user telephone is needed, thus, reducing the cost of the telephone service. In addition, the PBX typically provides a variety of features to the users of the PBX, such as connecting calls among the users and providing voicemail services.
To implement all of the functions of the PBX, certain control and overhead communications must take place between each user""s telephone set and the PBX. These communications typically include digital status and command signals in addition to the two-way voice signals necessary to carry on a telephone conversation. For example, the PBX must know whether a telephone set is connected to a particular extension in order to know whether or not to route calls to that extension. As another example, the PBX must interact with the user telephone sets in order for the users to initiate and terminate telephone calls and to access voicemail and other features of the PBX.
In general, the protocol utilized for control and overhead communications differs among the various manufacturers of PBXs. In addition, the voice signals can be digitally sampled and compressed according to various different companding schemes (e.g., xcexc-law or A-law). Therefore, a two-wire analog telephone instrument, such as modem, fax modem, facsimile machine or teleconferencing device, cannot generally interface directly with the PBX without a dedicated analog port from the PBX.
This creates a problem for users of a PBX who wish to use universally available analog telephone instruments, such as modems, fax modems, facsimile machines, teleconferencing devices, headsets or handsets, in addition to their PBX-compatible telephone sets. This problem has intensified by a recent increase in demand for access to the world wide web, which is typically accessed through use of a modem connected to a personal computer. A proposal has been to provide a dedicated outside line for each such analog telephone instrument. This solution is not entirely satisfactory, however, because it negates the savings which result from the PBX limiting the number of required outside lines. Another solution has been to provide an analog line card in the PBX and a separate line connecting the two-wire analog telephone instrument to the PBX. This solution can be costly due to the need to install separate extension lines to connect each of the PBX-compatible telephone set and the analog telephone instrument to the PBX. Further, the addition of analog line cards can necessitate a PBX that is has a higher capacity, and hence, higher cost, that would otherwise be required.
Another solution has been to provide a device which interfaces a modem with a telephone set through the handset port of the telephone set. For example, U.S. Pat. No. 4,907,267 discloses a modem interface device for use with a telephone set having a base unit and a handset. The telephone set can be a two-wire telephone set or a telephone set designed for use with a PBX. To use the modem interface device, the handset is unplugged from the handset jack of the base and plugged into a handset jack in one end of the device. Extending from the device is a four-wire cable which is connected to the handset jack of the base. The device also includes a modular jack for accepting a two-wire cable which connects the device to a two-wire telephone instrument, such as a modem. A series of switches are manually positioned to select between voice and data communications and to configure the interface device to match the signalling characteristics of the particular telephone set being used.
The manually operable switch arrangement described in U.S. Pat. No. 4,907,267 is improved upon in two products manufactured by Unlimited Systems Corp. of San Diego, Calif. A first of these products, the xe2x80x9cKONEXX Office Konnector,xe2x80x9d connects to the base of a telephone set and to the handset to provide an interface for a two-wire telephone, facsimile machine or modem. The device detects when the two-wire telephone, facsimile machine or modem is placed off-hook for switching between voice and data communications. A second of the these products, the xe2x80x9cKONEXX Konference,xe2x80x9d is similarly connected between the base and handset, but provides an interface for a teleconferencing device. For each of these devices, a manually operable switch is positioned in one of four positions for adjusting the device to the signalling characteristics of the particular telephone set being used.
A drawback to the aforementioned interface devices is that the switch positions may be incorrectly set. In addition, there is a likelihood of encountering a telephone set having signalling characteristics that cannot be met by these interface devices. For example, the bias voltage level and AC signal coupling characteristics of the microphone connection to the telephone base can vary from manufacturer to manufacturer as well as the output impedance and signal levels provided via the handset speaker connection. Also, the wire assignments within the handset cable can vary. A single return may be provided for both the microphone and speaker or each may have two dedicated wires. Failure of an interface device to properly match the signalling characteristics of the telephone set may result in inoperability or lost data and may result in the frustration of users of such devices who may not have the technical ability, nor the inclination, to resolve such problems.
What is needed is a telephone handset interface that has sufficient flexibility to match the signalling characteristics of a wide variety of commercially available telephone sets while minimizing technical ability required from a user of such interface.
The invention is a handset port interface including DC measurement and transmit path calibration. In an interface device for interfacing a telephone instrument, such asia headset, a handset, a modem, or a teleconferencing device, to the handset microphone input port of a telephone set base unit, the transmit signal level applied to the microphone input of the base unit must be set to an appropriate level. In accordance with the present invention, the appropriate level is determined according to the type of microphone the telephone set base unit is configured to interface with. For example, the microphone can be carbon, electret or dynamic. The interface device detects the type of microphone the base unit is configured to interface with by detecting the DC characteristics of the microphone input port. Then, the interface device determines the appropriate transmit level by correlating the DC characteristics to one of a plurality of predetermined transmit levels. These levels can be stored in a table. The transmit signal level is then automatically set according to a selected one of the predetermined transmit levels.
In accordance with an aspect of the present invention, a method of interfacing a telephone instrument to a handset port of a telephone set comprises steps of: identifying a type of microphone the telephone set is configured to interface with; correlating the microphone type to an appropriate transmit level for signals transmitted from the telephone instrument to the telephone set via a transmit path; and calibrating the transmit path in accordance with the appropriate transmit level.
In accordance with another aspect of the present invention, an interface apparatus for interfacing a telephone instrument to a handset port of a telephone set comprises: a handset port interface for interfacing to a handset port of a telephone set wherein the handset port includes a microphone input; a telephone instrument interface for interfacing to a telephone instrument; a transmit path for communicating signals from the telephone instrument interface to the handset port interface; and a controller for detecting a DC characteristic of the microphone input of the handset port and for adjusting a transmit sensitivity of the transmit path according to the DC characteristic.
In a first alternate embodiment, a test signal is applied to the microphone port of the base unit to determine its input characteristics and, then, the transmit sensitivity is set based upon this determination. In accordance with an aspect of the first alternate embodiment of the present invention, an interface apparatus for interfacing a telephone instrument to a handset port of a telephone set comprises: a handset port interface for interfacing to a handset port of a telephone set wherein the handset port includes a microphone input; a telephone instrument interface for interfacing to a telephone instrument; a transmit path for communicating signals from the telephone instrument interface to the microphone input of the handset port interface; and a controller for applying a test signal to the microphone input of the handset port interface and for measuring a response to the test signal at the microphone input of the handset port interface wherein the controller adjusts a transmit sensitivity of the transmit path according to the response to the test signal.
In accordance with another aspect of the first alternate embodiment of the present invention, an interface apparatus for interfacing a telephone instrument to a handset port of a telephone set comprises: a handset port interface for interfacing to a handset port of a telephone set wherein the handset port includes a microphone input; a telephone instrument interface for interfacing to a telephone instrument; a transmit path for communicating signals from the telephone instrument interface to the microphone input of the handset port interface; a controller for applying a test signal to the microphone input of the handset port interface and for measuring a response to the test signal; and a table wherein the table stores a plurality of predetermined transmit sensitivity levels, each predetermined transmit sensitivity level stored in association with a corresponding possible response to the test signal wherein the controller selects a transmit sensitivity for the transmit path locating an entry in the table corresponding to the response to the test signal.
In a second alternate embodiment, a test signal is applied to the microphone located in the handset to determine its type or characteristics and, then, the transmit sensitivity is set based upon this determination. In accordance with an aspect of the second alternate embodiment of the present invention, an interface apparatus for interfacing a telephone instrument to a handset port of a telephone set comprises: a handset port interface for interfacing to a handset port of a telephone set wherein the handset port includes a microphone input; a telephone instrument interface for interfacing to a telephone instrument; a transmit path for communicating signals from the telephone instrument interface to the microphone input of the handset port interface; a handset interface for interfacing to a handset having a microphone; and a controller for applying a test signal to the microphone of the handset and for measuring a response to the test signal wherein the controller adjusts a transmit sensitivity of the transmit path according to the response to the test signal.
As a further enhancement to the aforementioned embodiments, receive sensitivity can be adjusted for signals received from a handset speaker output port.